The present invention relates to the field of transducers, and more particularly to cylindrical PVDF ultrasonic airborne transducers.
In the environment of ultrasound transducers, it is known that a curved transducer made of a polymer piezoelectric material such as PVDF and clamped at both ends may be used to form an audio or ultrasonic air transducer. Numerous examples of such may be found in the prior art references, such as M. Tamura, T. Yamaguchi, T. Oyabe and T. Yoshimi xe2x80x98ELECTROACOUSTIC TRANSDUCERS WITH PIEZOELECTRIC HIGH POLYMER FILMSxe2x80x9d, J. Audio Eng. Soc. Vol. 23, No. 1, pp 21-26, (1975); R. Lerch and G. M. Sesler, xe2x80x98MICROPHONES WITH RIGIDLY SUPPORTED PIEZOPOLYMER MEMBRANExe2x80x9d, J. Acoust. Soc. Am. Vol. 67, No. 4, pp 1379-81, (1980); Jeff S. Schoenwald and Jim. F. Martin, xe2x80x9cPVF2 TRANSDUCERS FOR ACOUSTIC PANNING AND IMAGING IN AIR.xe2x80x9d, 1983 Ultrasonic Symposium (IEEE), pp. 577-580; F. Harnisch, N. Kroemer, and W. Manthey, xe2x80x9cULTRASONIC TRANSDUCERS WITH PIEZOELECTRIC POLYMER FOILxe2x80x9d, Sensors and Actuators A25-27, xe2x80x94549-552 (1991); S. Edelman and A. S. DeReggi xe2x80x9cCOMMENTS ON ELECTROACOUSTIC TRANSDUCERS WITH PIEZOELECTRIC HIGH POLYMER FILMSxe2x80x9d, J. Aoudio Eng. Soc vol. 24, No 7, pp. 577-578, (1976); I. Veit xe2x80x9cTHE PIEZOELECTRIC PVDF-FILM-ITS PROPERTIES AND APPLICATION IN ELECTROACOUSTIC TRANSDUCERSxe2x80x9d, Audio Eng. Soc., 84th Convention March 1988 1-4 paris 2604 (G-1); A. S. Fiorillo xe2x80x9cDESIGN AND CHARACTERIZATION OF A PVDF ULTRASONIC RANGE SENSORxe2x80x9d IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control, vol. 39, No. 6, pp. 688-692(1992); R. Lerch, xe2x80x9cELECTROACOUSTIC TRANSDUCER USING PIEZOELECTRIC POLYVINYLIDENE FLUORIDE FILMS.xe2x80x9d, J. Acoust. Soc. Am Vol. 66, No. 4, pp. 952-954 (1979); W. Flugge xe2x80x9cstatik und Dynamik der Schalenxe2x80x9d Springer, Berlin 1962; Hong Wang and Minoru Toda, xe2x80x9cCurved PVDF Airborne Transducerxe2x80x9d, to be published; Leo L. Beranek xe2x80x9cAcousticsxe2x80x9d, The American Institute of Physics, p. 119. 1986.
Referring now to FIG. 1A, there is shown a cylindrical piezoelectric film 54 having its stretched axis wrapped around a cylinder (not shown). Here, when a AC voltage is applied to electrodes 56 on surfaces of the cylindrical film 54, a lengthwise strain in the curved direction is converted to a film displacement normal to the surface (or vice-verse), due to the cylindrical film structure. Thus, a lengthwise strain in the curved direction is converted to radial vibration. Such a structure can be used as either a transmitter or a receiver with omni-directional angle performance. Depending on the application, often it is necessary to have transducers with limited angle performance (narrower directivity). In such a case, conventional transducers use two end clamped curved film structures as shown in FIG. 1B. However, in the known application of a curved film with two clamps, two significant problems are present. First, the resonance of the housing 22 on which the clamp is attached, reduces the stiffness of the clamp. One of many resonance modes of the housing structure are often coincident to the main resonance frequency of the curved film. The resonance of the curved film requires a very stiff clamp structure. Control of the housing resonance is thus very difficult and very sensitive to any minor variation of housing dimension, such that the output or sensitivity of each device is not reproducible and entirely non-uniform. Secondly, the thermal expansion coefficient of PVDF film is very high (approximately 120xc3x9710xe2x88x926/C, where the metal has a value of between 10xc3x9710xe2x88x926/C and 20xc3x9710xe2x88x926/C, 10xcx9c20xc3x9710xe2x88x926/C). At relatively high temperatures (above approximately 45 C. for example) thermal expansion of the film severely deforms the film shape. This is because the clamp material, which has a much lower expansion causing deformation of the film and thereby creating film buckling around the cylinder. Once such buckling occurs, the film shape can not be restored to its original shape, even after the PVDF film is allowed to return to normal ambient temperature conditions.
In order to overcome these problems with clamped transducer structures, a non-clamp structure is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/281,398 entitled OMNI-DIRECTIONAL ULTRASONIC TRANSDUCER APPARATUS HAVING CONTROLLED FREQUENCY RESPONSE and co-pending and commonly assigned U.S. patent application Ser. No. 09/281,247 entitled OMNI-DIRECTIONAL TRANSDUCER APPARATUS AND STAKING METHOD disclose such non-clamped structures, the subject matter of which is incorporated herein by reference in their entireties. These documents disclose a non-clamped omni-directional transducer comprising a cylindrical PVDF film wrapped around a spool, where the film is spaced apart from a body portion of the spool to form a gap sized to enable the resonance frequency of the transducer to be controlled by the resonance frequency of the piezoelectric film.
In the above disclosures, the PVDF film included an electrode layer deposited over substantially the entire front surface of the film and a second electrode layer deposited on substantially the entire back surface of the PVDF film, except for the peripheral edges of the film in order to facilitate bonding. The angular performance of acoustic properties of PVDF transducers disposed onto a spool (either clamped or unclamped) is omni-directional. However, depending on the type of application, it is sometimes desirable to limit directivity of the beam angle to within a certain range. It is further desirable in certain instances, such as in the detection of low SNR signals, to obtain a receiver having increased sensitivity for detecting such signals. Still further, certain applications may make use of a wide band transducer having a relatively low Q factor and wider useful common frequency band between transmitter and receiver. A transducer capable of solving the aforementioned problems is highly desired.
The present invention modifies the structure of a cylindrical PVDF film material to obtain a desired angle performance.
It is an object of the present invention to provide an air transducer apparatus comprising a curved piezoelectric film having a first surface and a second surface opposite the first surface, a first electrode layer disposed on at least a portion of the first surface of the film; and a second electrode layer disposed on a portion of the second surface of the film defining the second electrode layer being sized less than the first electrode layer for generating a signal in response to acoustic energy incident on the piezoelectric film.
It is a further object of the present invention to provide an acoustic receiver comprising a cylindrical piezoelectric film having a first surface and a second surface opposite the first surface, the film responsive to acoustic energy incident thereon for vibrating at a given frequency; a first electrode layer disposed on the first surface of the film; a second electrode layer disposed on a portion of the second surface of the film; and a housing surrounding the piezoelectric film, the housing formed of a material resistant to the propagation of the acoustic waves, the housing having an aperture in alignment with at least a portion of the second electrode covering the film for influencing a receiving beam angle associated with acoustic radiation incident on the piezoelectric film through the aperture.
It is still a further object of the present invention to provide an acoustic transmitter comprising a cylindrical piezoelectric film having a first surface and a second surface opposite the first surface; a first electrode material disposed on the first surface of the film; a second electrode material disposed on the second surface of the film; means for exciting the film to generate omnidirectional acoustic waves at a resonance frequency; and a housing surrounding the piezoelectric film and formed of a material resistant to the propagation of the acoustic waves, the housing having an aperture exposing a portion of the second electrode covering the film to provide a directed acoustic wave output from the transducer.
Still further, it is an object of the present invention to provide a wideband transducer comprising a frame having a substantially cylindrical body portion, a substantially cylindrical piezoelectric film surrounding at least part of the frame body portion, the film spaced apart from the body portion to form a predetermined gap between the film and the body, the film having a first electrode layer disposed on a first surface of the film, and a second electrode layer disposed on a portion of a second surface of the film opposite the first surface. A loss of material is positioned within the predetermined gap and the film is excited so as to cause the film to vibrate at a resonance frequency. The resonance frequency becomes broader due to the loss of material which fills the gap.